Spiritual growth is pursuing and growing in the character and holy nature of Jesus Christ in one’s thoughts, feelings, deeds, actions, and reactions.

Desire the sincere milk of the Word to grow thereby.

As newborn babes, desire the sincere milk of the word, that ye may grow thereby
— I Peter 2:2
But speaking the truth in love, may grow up into him in all things, which is the head, even Christ
— Ephesians 4:15
In whom all the building fitly framed together groweth unto an holy temple in the Lord
— Ephesians 2:21

We are to go on unto perfection or spiritual maturity.

Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection
— Hebrews 6:1

The evidence of spiritual growth is the manifestation of the Nature of God’s Son in the believer’s life, along with the fruit of the Spirit.

But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, Meekness, temperance: against such there is no law. And they that are Christ’s have crucified the flesh with the affections and lusts
— Galatians 5:22-24
(For the fruit of the Spirit is in all goodness and righteousness and truth;)
— Ephesians 5:9

Jesus is our example of how to be and how to do.

For I have given you an example, that ye should do as I have done to you
— John 13:15
For even hereunto were ye called: because Christ also suffered for us, leaving us an example, that ye should follow his steps
— I Peter 2:21

Growing in God is following the example of Jesus Christ in all things.

And I, brethren, could not speak unto you as unto spiritual, but as unto carnal, even as unto babes in Christ. I have fed you with milk, and not with meat: for hitherto ye were not able to bear it, neither yet now are ye able. For ye are yet carnal: for whereas there is among you envying, and strife, and divisions, are ye not carnal, and walk as men?
— I Corinthians 3:1-3

Paul informed the Corinthian church that while there remained un-Christ-like attitudes and dispositions in their midst, they were immature and carnal. Spiritual maturity expresses the Nature of Christ.

One of the greatest pictures of the need for Christians to grow in the Nature and Character of Jesus Christ is in the Parable of the Sower taught by Jesus Himself in Matthew Chapter 13. Here the truth is plainly stated that the Seed is the Word of God planted in the believer’s heart when he or she is saved. Because God provides the sunlight and rain the Seed will grow, but there are many obstacles to the growth of the Word in man’s heart that must be overcome. Those that do overcome and allow the Word of God to grow within their hearts will produce fruit some 30 fold, some 60 fold and some will grow all the way to 100 fold. These levels of growth are pictures of the maturity that the believer reaches as his or her heart is transformed into the character and nature of Jesus Christ.

Our Christian growth is determined by how much of the fruit of the Spirit we have growing on the Tree of Life within us. True Christian growth is not measured by our works of ministry or good deeds.

But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, Meekness, temperance: against such there is no law
— Galatians 5:22-23

Never has a farmer planted his seed and gone home rejoicing that the work was done and the harvest accomplished. Rather, the farmer knows that once the seed is planted, the work has just begun. There is much the believer has to do in order for the Word of God to continue to grow in his or her heart.